The invention relates to a seal for sealing an interface between a piston shaft of an expander mechanism, for example in the form of an adjustable piston, for a disk brake, and a laterally inner surface of a housing of the brake in which the shaft is received.
Seals of the type described in the last preceding paragraph are known which have a laterally inwardly disposed portion adapted to form a wiper seal with an outer surface of the piston shaft. In use, a piston shaft inserted in the known type of seal moves along a longitudinal axis of the shaft relative to the laterally inwardly disposed portion of the seal to accommodate regular movement of the piston shaft during operation of the brake and also to accommodate movement of the piston shaft caused by lengthening of the expander mechanism to compensate for brake friction element wear. In this connection, a piston shaft generally moves axially by about 1 to 2 mm, for example, during normal braking, and by up to about 20 mm, for example, during adjustment of expander mechanism length to compensate for brake friction element wear.
The laterally inwardly disposed seal portion which forms the wiper seal is subjected to a significant amount of wear. Furthermore, the seal materials suitable to satisfy the operational requirements of wiper seals generally have a lesser ability to withstand high temperatures. This is significant in the operational environment to which disk brake expander mechanisms are exposed, where high temperatures are common because of the proximity of brake friction elements to the expander mechanism.
Seals of the above described type, which engage a laterally outer surface of a piston shaft, in use, are generally provided in the form of secondary seals, to prevent dirt and debris from jamming the expander mechanism in the event of failure of a primary seal which is generally provided between a piston head of the piston assembly and the brake housing in which the piston shaft is received.
According to the invention, there is provided a seal for sealing an interface between a piston shaft of an expander mechanism for a disk brake and an inner surface of a housing of the brake in which the shaft is received, the seal comprising:
a laterally outwardly disposed portion adapted to engage a portion of the housing in fixed relation therewith;
a laterally inwardly disposed portion adapted to form a static seal with a surface of the shaft; and
an intermediate portion interconnecting said laterally outwardly disposed portion and said laterally inwardly disposed portion such that the laterally inwardly disposed portion is permitted to move axially together with the shaft relative to the laterally outwardly disposed portion, in use, in static sealing relationship with the shaft.
This facilitates provision of a static seal with the shaft during regular movement of the shaft relative to the housing for accommodating normal braking movement of the shaft, and avoids the necessity for a conventional wiper seal to accommodate such movement.
The invention is particularly advantageous in that it facilitates the reduction of seal wear against the piston shaft caused by regular brake operation, and/or in that it facilitates the use of a material with a higher temperature capability for fabrication of the seal than would be possible if a conventional wiper seal were provided for engaging with the piston shaft. Furthermore, a more secure seal can be provided with the piston shaft compared to conventional wiper seals, which tend to wear and lose interference on the piston shaft. Still further, less stringent design constraints are placed on the surface of the piston shaft outer surface.
The laterally inwardly disposed portion of the seal may also be adapted to accommodate occasional axial sliding movement of the laterally inwardly disposed portion of the seal relative to the piston shaft, in use, without losing seal integrity. Such axial sliding movement can occur, for example, during lengthening of the expander mechanism in order to compensate for wear of wearable friction elements of the brake, whereby the rest position of the piston shaft is adjusted axially relative to the housing. This facilitates restricting the occurrence of such sliding movement and consequential seal wear to occasions of large relative movement such as that caused by lengthening of the expander mechanism, and the avoidance of sliding during the many small movements made by the piston shaft during normal brake applications. With certain seal designs in accordance with the invention, relative sliding between the laterally inwardly disposed portion of the seal and the piston shaft might also be expected to occur during very hard brake applications, where travel of the piston shaft exceeds that experienced during the vast majority of brake applications.
The intermediate portion may comprise a flexible web. The web may be adapted to permit bellows-like lateral flexure of the web when a piston shaft is disposed through the seal in an unadjusted condition of the adjustable expander mechanism.
The web may be of substantially uniform thickness.
Preferably, lateral dimensions of the intermediate portion are chosen to be significantly greater than the minimum lateral distance, in use, in the unadjusted condition of the expander mechanism, between the laterally outwardly disposed portion and the laterally inwardly disposed portion.
The laterally inwardly disposed portion may comprise a rim having an axially and laterally inwardly disposed bevel for facilitating insertion of a piston shaft through the seal, in use.
The laterally outwardly disposed portion may comprise a relatively thin, laterally outer, axially extending, extension of the intermediate portion, bonded to a substantially rigid laterally outermost member.